1. Field of the Invention
This invention relates to an earthquake-insulating bearing assembly for mounting between a foundation and a building structure such as a nuclear reactor building, a bridge and the like so as to efficiently protect it from an earthquake attack.
2. Prior Art
One conventional earthquake-insulating bearing 50 shown in FIG. 1 is of an elastic construction comprising elastic sheets 51 and metal sheets 52 stacked one upon another alternately and connected together. A plurality of bearings 50 are mounted between a foundation 53 and a building structure 54. When an earthquake occurs, the foundation 53 is moved horizontally so that the lower portion of the elastic bearing 50 is also moved horizontally together with the foundation 53. At this time, the elastic bearing 50 is elastically deformed to suitably absorb a horizontal inertia force of the building structure 54 to thereby protect it from an earthquake attack. This elastic bearing may be of a one-piece molded construction comprising a plurality of closely spaced metal sheets 52 molded in rubber or the like, as shown in FIG. 2. Such conventional elastic bearings have been found not satisfactory, however, in that when a severe earthquake occurs, the elastic bearings are elastically deformed to such an extent that the horizontal displacement of the building structure 54 relative to the foundation 53 is excessive to frequently damage a piping installed between the building structure 54 and the ground. In addition, in such a case, the elastic bearings themselves can be subjected to damage.
In order to overcome the above-mentioned difficulty, another earthquake-insulating device 56 shown in FIGS. 3A to 3C has been proposed in the art. An elastic bearing 57 in FIGS. 3A to 3C is identical in construction to the bearing 50 in FIG. 2. A friction plate 58 is fixedly secured to an upper end of the elastic bearing 57, and a friction plate 59 is fixedly secured to a lower surface of the building structure 54. The friction plate 58 is held in frictional contact with the friction plate 59. With this construction, a horizontal inertia force of the building structure 54 is absorbed both by the elastic deformation of the elastic bearing 57 and the friction between the two friction plates 58 and 59 during an earthquake. With this earthquake-insulating device, however, when an earthquake occurs, the friction plate 59 is horizontally displaced or moved relative to the friction plate 58 as shown in FIG. 3B, and after the earthquake is over, the friction plate 59 is retained in its displaced position as shown in FIG. 3C due to the friction between the two friction plates 58 and 59. As a result, the building structure 54 is returned to its initial position (FIG. 3A) using jacks or other lifting devices. This requires much time and labor. In addition, when a severe earthquake occurs, there is a risk that the friction plate 58 is brought out of engagement with the friction plate 59. Further, it is also possible that the elastic bearing 57 is caused to fall down.
FIG. 4A shows a further conventional earthquake-insulating assembly 60. This assembly comprises an elastic bearing 61 of the type described above and a flexural beam 62 fixedly secured at opposite ends thereof to a support member 63 and an engaging member 64 on the building structure 54, respectively. With this arrangement, a horizontal inertia force of the building structure 54 is absorbed both by the elastic deformation of the elastic bearing 61 and the flexural deformation of the beam 62. A hysteresis loop of the flexural beam 62 is shown in FIG. 4B, and the inclination of the curve representative of the relation between the force F on the weight and its displacement is relatively large. Therefore, a horizontal soft stiffness of the elastic bearing 61 is affected by a modulus of elasticity in an initial deformation of the beam 62. As a result, the elastic bearing can not fully achieve its intended function. Further, the hysteresis of the beam 62 is obtained by its flexural deformation, and therefore the beam 62 can not be installed in a space between the foundation 53 and the building structure 54. This necessitates the use of the support member 63 of the beam 62, and therefore the installation of this earthquake-insulating assembly 60 can not be carried out so easily. Further, after an earthquake is over, the beam remains deformed. As a result, the building structure 54 must be brought into its initial position using jacks or the like. Alternatively, it is necessary to remove the deformed beam for replacement with a new one to achieve this.
A further conventional earthquake-insulating assembly 65 shown in FIG. 5 comprises an elastic bearing 66 of the type described above and an oil damper cylinder 67. A connecting member 68 is mounted on a lower surface of the building structure 54. The upper end of the elastic bearing 66 is fixedly secured to the connecting member 68. A piston rod 67a of the oil damper cylinder 67 is connected to the connecting member 68. With this construction, a horizontal inertia force of the building structure 54 is absorbed both by the elastic bearing 66 and the oil damper cylinder 67. The oil damper cylinder 67 has a speed dependency and therefore is not so effective for absorbing vibrations having a relatively long cycle, such as those caused by an earthquake. Further, the oil damper cylinder can absorb displacement of the building structure only in an axial direction of the piston rod 67a, and is not effective for displacement in the other directions.